Editing Schwann cell (section)

==Function==
The [[vertebrate]] [[nervous system]] relies on the myelin sheath for [[Insulator (electrical)|insulation]] and as a method of decreasing membrane [[capacitance]] in the axon. The [[action potential]] jumps from node to node, in a process called [[saltatory conduction]], which can increase [[Electrical conduction|conduction]] velocity up to 10 times, without an increase in axonal diameter. In this sense, Schwann cells are the PNS's analogues of the [[central nervous system]]'s [[oligodendrocyte]]s. However, unlike oligodendrocytes, each myelinating Schwann cell provides insulation to only one axon (see image). This arrangement permits saltatory conduction of action potentials with [[wiktionary:propagation|repropagation]] at the nodes of Ranvier. In this way, myelination greatly increases speed of conduction and saves energy.<ref>Kalat, James W. ''Biological Psychology'', 9th ed. US: Thompson Learning, 2007.{{Page needed|date=April 2011}}</ref>

Nonmyelinating Schwann cells are involved in maintenance of axons and are crucial for neuronal survival. Some group around smaller axons ([http://www.liv.ac.uk/~rbj/RBJ/rbjteaching/Schwann%20Cells.htm External image here]) and form [[C fiber#Remak bundles|Remak bundles]].

Myelinating Schwann cells begin to form the myelin sheath in mammals during fetal development and work by spiraling around the axon, sometimes with as many as 100 revolutions.  A well-developed Schwann cell is shaped like a rolled-up sheet of paper, with layers of myelin between each coil. The inner layers of the wrapping, which are predominantly [[cell membrane|membrane]] material, form the myelin sheath, while the outermost layer of nucleated [[cytoplasm]] forms the [[neurilemma]]. Only a small volume of residual cytoplasm allows communication between the inner and outer layers. This is seen [[Histology|histologically]] as the [[Schmidt-Lantermann incisure]].

=== Regeneration ===
Schwann cells are known for their roles in supporting [[nerve regeneration]].<ref>{{cite journal |pages=1995–99 |doi=10.1016/j.biocel.2006.05.007 |title=Schwann cells: Origins and role in axonal maintenance and regeneration |year=2006 |last1=Bhatheja |first1=Kanav |last2=Field |first2=Jeffrey |journal=The International Journal of Biochemistry & Cell Biology |volume=38 |issue=12 |pmid=16807057}}</ref> Nerves in the PNS consist of many axons myelinated by Schwann cells. If damage occurs to a nerve, the Schwann cells aid in digestion of its axons ([[phagocytosis]]). Following this process, the Schwann cells can guide regeneration by forming a type of tunnel that leads toward the target neurons. This tunnel is known as [[band of Büngner]], a guidance track for the regenerating axons, which behaves like an endoneural tube. The stump of the damaged axon is able to sprout, and those sprouts that grow through the Schwann-cell "tunnel" do so at the rate around 1&nbsp;mm/day in good conditions. The rate of regeneration decreases with time. Successful axons can, therefore, reconnect with the muscles or organs they previously controlled with the help of Schwann cells, but specificity is not maintained and errors are frequent, especially when long distances are involved.<ref>Carlson, Neil R. ''Physiology of Behavior'', 9th ed. US: Pearson Education, Inc., 2007.{{Page needed|date=April 2011}}</ref> Because of their ability to impact regeneration of axons, Schwann cells have been connected to [[preferential motor reinnervation]], as well. 
If Schwann cells are prevented from associating with axons, the axons die. Regenerating axons will not reach any target unless Schwann cells are there to support them and [[Axon guidance|guide]] them. They have been shown to be in advance of the [[growth cone]]s.  
Schwann cells are essential for the maintenance of healthy axons. They produce a variety of factors, including [[neurotrophin]]s, and also transfer essential molecules across to axons.[[Image:Cultured schwann cell.jpg|thumb|left|alt=Schwann cell.|A Schwann cell in culture]]